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javalambdajava-8functional-interface

Why we can't assign lambda expression directly to reference variable of type Object?

I am doing some experiment with lambda expression and saw some behavior which I can't understand. 

    Consumer consumer = (o1) -> {};
    Object obj1 = consumer; // this two line working fine

Above 2 lines of code does not complain anything as expected when I assign consumer to obj1.

However when I tried to assign directly the lambda to object it started giving me a compilation error. 

    Object obj2 = (o1) -> {}; // this line gives compilation error

The above line of code gives me an error :

The target type of this expression must be a functional interface.

My question is why we can't directly assign a lambda to a reference variable of type Object?

Edited: I have edited my question as there is a similar question already mentioned but my question main goal was to ensure why Object o1 = "Hello" will work but not the lambda.

Solution

  • If you assign a lambda expression to a variable of type Object, the compiler has no idea which functional interface this lambda expression should implement.

    For example, the lambda expression o -> o.toString () can be assigned to either a Consumer or a Function:

    Consumer<String> cons = o -> o.toString ();
Function<String,String> cons2 = o -> o.toString ();

    Therefore you have to either assign the lambda expression to a variable of a functional interface type or to cast the lambda expression to some functional interface type before assigning it to the Object variable.